Magnetic recording medium

ABSTRACT

A magnetic recording medium is disclosed, which comprises a nonmagnetic support having thereon a magnetic layer containing at least ferromagnetic metal particles, wherein said ferromagnetic metal particles have a coercive force of from 1,700 to 3,000 Oe, an average long axis length of from 30 to 80 nm, an average acicular ratio of from 2.0 to 5.0, and a crystallinity of from 30 to 100%.

FIELD OF THE INVENTION

The present invention relates to a magnetic recording medium, e.g., amagnetic tape. More particularly, this invention relates to a coatingtype magnetic recording medium which has a magnetic layer formed bycoating on a nonmagnetic support a magnetic coating compositioncontaining ferromagnetic metal particles and a binder as majorcomponents, and which is excellent in output, C/N, and overwritingcharacteristics in a short-wavelength region.

BACKGROUND OF THE INVENTION

The technique of magnetic recording has excellent advantages over otherrecording systems, for example, that a recording medium can berepeatedly used, that signal conversion into electronic signals is soeasy that a magnetic-recording apparatus can be combined with peripheraldevices to construct a system, and that signals can be easily revised.Due to such advantages, the magnetic recording has been widely utilizedin various fields including video, audio, and computer applications.

With respect to recording media, a further improvement in recordingdensity, reliability and durability has been always desired so as tocope with desires for size reduction in appliances, improvement in thequality of reproduced signals, elongation of recording time, and anincrease of recording capacity.

For example, magnetic recording media for use in audio and videoapplications have come to be required to have suitability for therecording/reproduction of signals having an even shorter wavelength thanin conventional systems and to be excellent in reliability anddurability even at an increased head/medium relative speed, in order tocope with practical use of the digital recording system attainingimproved sound and image quality and with the development of a videorecording system for high-definition television.

Also, in computer applications, it is desired to develop alarge-capacity digital recording medium for storing an increasingquantity of data therein.

For the attainment of a higher recording density in coating typemagnetic recording media, various methods have been investigated andproposed, for example, from the standpoint of obtaining a magnetic layerhaving improved magnetic characteristics by replacing conventionalmagnetic iron oxide particles with magnetic particles of iron or analloy mainly comprising iron or by using magnetic particles withimproved magnetic characteristics, e.g., finer magnetic particles, andimproving the filling property and orientation property of thesemagnetic particles, and from the standpoints of improving thedispersibility of ferromagnetic particles and enhancing the surfaceproperties of a magnetic layer.

For example, a technique of using ferromagnetic metal particles or ahexagonal ferrite as ferromagnetic particles in order to enhancemagnetic characteristics is disclosed in, e.g., JP-A-58-122623 (the term"JP-A" as used herein means an "unexamined published Japanese patentapplication"), JP-A-6174137, JP-B-62-49656 (the term "JP-B" as usedherein means an "examined Japanese patent publication"), JP-B-60-50323,and U.S. Pat. Nos. 4,629,653, 4,666,770, and 4,543,198.

JP-A-1-18961 discloses ferromagnetic metal particles having a long axislength of 0.05 to 0.2 μm, an acicular ratio of 4 to 8, a specificsurface area of 30 to 55 m² /g, a coercive force (Hc) of 1,300 Oe ormore, and a saturation magnetization (σ_(s)) of 120 emu/g or more. Thistechnique is to provide fine metal particles having a small specificsurface area.

JP-A-60-11300 and JP-A-60-21307 disclose a process for producing fineacicular crystals of α-iron oxyhydroxide which are suitable for use inproducing ferromagnetic particles, in particular, ferromagnetic metalparticles. The latter reference discloses that ferromagnetic metalparticles having an Hc of 1,450 to 1,600 Oe and a σs of 142 to 155 emu/gare produced from goethite having a long axis length of 0.12 to 0.25 μmand an acicular ratio of 6 to 8.

JP-A-6-340426 and JP-A-7-109122 disclose monodisperse spindle-shapedhematite particles obtained from hematite nuclei, iron hydroxide, andspecific ions and extremely fine ferromagnetic particles obtained byreducing the hematite particles.

It has also been proposed to use various surfactants (as disclosed in,e.g., JP-A-52-156606, JP-A-53-15803, JP-A-53-116114) and variousreactive coupling agents (as disclosed in, e.g., JP-A-49-59608,JP-A-56-58135, JP-B-62-28489) for enhancing the dispersibility offerromagnetic particles.

JP-A-1-239819 discloses magnetic particles obtained by successivelyadhering a boron compound and an aluminum compound or an aluminumcompound and a silicon compound to the surface of magnetic iron oxideparticles. This prior art technique is to improve magneticcharacteristics and dispersibility.

JP-A-7-22224 discloses ferromagnetic metal particles in which thecontent of Group 1a elements of the Periodic Table is 0.05% by weight orless and which may contain aluminum and a rare earth element in amountsof 0.1 to 30% and from 0.1 to 10%, respectively, in terms of the amountof the atoms thereof based on the total amount of all metallic elementsand may have a content of residual Group 2a elements of the PeriodicTable of 0.1% by weight or less. This technique is to provide ahigh-density magnetic recording medium having good storage stability andexcellent magnetic characteristics.

Furthermore, a technique of treating the surface of a magnetic layerafter coating and drying for improving the surface properties of themagnetic layer has been proposed (as disclosed in, e.g., JP-B-60-44725).

Strenuous efforts are being made to use signals having shorterwavelengths in order to attain higher recording densities in magneticrecording media. In a magnetic recording medium in which the length of aregion used for recording a signal has become a size comparable withthat of the magnetic particle used, recording is virtually impossiblebecause a distinct magnetization transition state cannot be created.Consequently, it is necessary to develop a magnetic particle having aparticle size sufficiently smaller than the shortest wavelength to beused, and investigations have long been direct to particle sizereduction in magnetic particles.

With respect to metal particles for magnetic recording, an acicularparticle shape is employed to impart shape anisotropy to thereby obtaina desired coercive force. It is well-known to persons skilled in the artthat for attaining higher-density recording, ferromagnetic metalparticles should be finely divided into finer particles so as to obtaina medium having diminished surface roughness. However, the metalparticles for use in magnetic recording tend to have a lower acicularratio with decreasing particle size and, as a result, come to beincapable of having a desired coercive force. A DVC system in whichvideo signals are recorded as digital signals was proposed recently, forwhich a high-performance ME tape and a high-performance MP tape areused. Since the MP tape for use in DVC has a coercive force of 2,000 Oeor higher, it is necessary to employ fine ferromagnetic metal particleshaving a high coercive force and an excellent particle sizedistribution. Moreover, since DVC is a recording system in which signalsare recorded over signals which have been recorded, the ferromagneticmetal particles for use therein are required to have satisfactoryoverwriting characteristics.

The applicant previously proposed ferromagnetic metal particles suitablefor use in a DVC system and a magnetic recording medium containing theferromagnetic metal particles (JP-A-7-326035), for the purpose ofproviding a magnetic recording medium having a magnetic layer regulatedso as to have a coercive force of 2,000 to 3,000 Oe, a thickness of 0.05to 0.3 μm, and a surface roughness of 1 to 3 nm and to have a specificcontent of components undergoing reversal of magnetization.

SUMMARY OF THE INVENTION

The present invention is connected with the prior art techniquedescribed just above, and provides a means for further improving theuniformity in performance and quality of the prior art magneticrecording medium.

The present invention has been achieved in view of problems of the priorart techniques described above.

An object of the present invention is to provide a magnetic recordingmedium which has satisfactory short-wavelength output and SIN andexcellent overwriting characteristics and is applicable to ahigh-density digital recording system.

The object of the present invention is accomplished with (1) a magneticrecording medium comprising a nonmagnetic support having thereon amagnetic layer containing at least ferromagnetic metal particles,wherein said ferromagnetic metal particles have a coercive force (Hc) offrom 1,700 to 3,000 Oe, an average long axis length of from 30 to 80 nm,an average acicular ratio (long axis/short axis) of from 2.0 to 5.0, anda crystallinity of from 30 to 100%. Also object is preferablyaccomplished with (2) a magnetic recording medium of the kind describedin (1) above which has a nonmagnetic layer mainly comprising inorganicnonmagnetic particles and a binder between the nonmagnetic support andthe magnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a ferromagnetic metal particle for use inthe present invention.

FIG. 2 is a view illustrating a conventional ferromagnetic metalparticle.

Description of the Symbols!

1 ferromagnetic metal particle

2 crystallite

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, the average long axis length of ferromagneticmetal particles means the average lengths of the long axes of theindividual particles, while the average short axis length thereof meansthe average lengths of the short axis of the individual particles. Theaverage acicular ratio thereof means the value obtained by dividingaverage long axis length by the average short axis length.

The crystallinity of ferromagnetic metal particles in the presentinvention means the proportion of particles each consisting of onecrystallite to all of from 200 to 300 grains, which are examined with ahigh-resolution electron microscope. The crystallites mean theindividual metal crystals of which the ferromagnetic metal particles arecomposed. The individual ferromagnetic metal particles, which each isdefined by the metal particle contour, need not consist of one crystaland may be composed of two or more crystallites. In a photograph takenwith a high-resolution transmission electron microscope, the particledefined by a largest particle contour is a ferromagnetic metal particle.Upon closer examination of the photograph, lattice images are obtained.The units which give the lattice images are crystallites. The acicularratio of a crystallite means the value obtained by dividing the longaxis length of the crystallite by the short axis length thereof.

According to the present invention, by regulating ferromagnetic metalparticles so as to have an average long axis length of from 30 to 80 nmand an average acicular ratio of from 2.0 to 5.0 and simultaneouslyregulating the ferromagnetic metal particles so as to have acrystallinity of from 30 to 100%, the coercive force (H_(c)) and theσ_(s) of the ferromagnetic metal particles can be regulated to from1,700 to 3,000 Oe and from 125 to 165, respectively. As a result, amagnetic recording medium having satisfactory short-wavelength outputand S/N and excellent overwriting characteristics can be provided.

In the present invention, the average long axis length of theferromagnetic metal particles is from 30 to 80 nm, preferably from 30 to70 nm, more preferably from 35 to 60 nm, in order to reduce the surfaceroughness of the magnetic recording medium. The average acicular ratioof the ferromagnetic metal particles is from 2.0 to 5.0, preferably from2.5 to 5.0. The crystallinity is from 30 to 100%, preferably from 35 to100%, more preferably from 40 to 100%.

Ferromagnetic metal particles having an average long axis length ofsmaller than 30 nm are undesirable since not only the desired coerciveforce cannot be obtained, but also such ferromagnetic metal particlesare dispersed with difficulty in the preparation of a magnetic coatingcomposition and are less effectively oriented in magnetic fieldorientation. In addition, due to the influence of the oxide film formedfor stabilization, it is difficult to ensure a high saturationmagnetization necessary for high-density recording. Ferromagnetic metalparticles having an average long axis length exceeding 80 nm areundesirable since H_(c) cannot be effectively heightened because acrystallinity of from 30 to 100% is obtained with difficulty and thereversal of magnetization does not occur by the simultaneous rotationmechanism.

A crystallinity of lower than 30% are undesirable from the standpoint ofoverwriting characteristics because not only such ferromagnetic metalparticles cannot have a high H_(c), but also the H_(c) distributionthereof is deteriorated considerably in particular, r3000/H_(c) (i.e.,(proportion of components undergoing reversal of magnetization at anH_(c) of 3,000 Oe or higher)/H_(c)) increases!. Although thecrystallinity of ferromagnetic metal particles should be from 30 to100%, it is preferably 40% to 100%. Ideal ferromagnetic metal particleshave a sufficiently low coefficient of variation of average long axislength (standard deviation of long axis length)/(average long axislength)! and an average acicular ratio of, in particular, from 3 to 5and have a crystallinity of 100%, because such ferromagnetic metalparticles have a high H_(c) and a narrow H_(c) distribution. When rawmaterials having the same composition were reduced under variousconditions to impart various crystallinity, ferromagnetic metalparticles having higher crystallinity tended to have lower contents ofhigh-coercive-force components.

The ferromagnetic metal particles used in the present invention have asaturation magnetization of generally from 125 to 165 emu/g, preferablyfrom 135 to 165 emu/g. A technique effective in obtaining ferromagneticmetal particles having a heightened saturation magnetization is toconduct, just after reduction, a treatment with any of compoundsdescribed in JP-A-61-52327 and JP-A-7-94310 and coupling agents havingvarious substituents, followed by gradual oxidation. The coercive forceof the ferromagnetic metal particles is from 1,700 to 3,000 Oe,preferably from 1,800 to 2,600 Oe. The present inventor presumes that byregulating ferromagnetic metal particles so as to have a crystallinityof from 30 to 100%, the mode of reversal of magnetization becomes closeto the simultaneous rotation mode as stated above and the ferromagneticmetal particles obtained can be fine particles having a high coerciveforce as in the present invention.

The magnetic layer of the present invention has a coercive force (H_(c))of usually from 1,800 to 3,000 Oe, preferably from 1,900 to 2,800 Oe,more preferably from 2,200 to 2,800 Oe, and a B_(m) (maximum fluxdensity) of usually from 3,500 to 5,500 gauss (G), preferably from 3,900to 5,500 G. If H_(c) or B_(m) is lower than the above lower limit,sufficiently high short-wavelength output cannot be obtained. If H_(c)or B_(m) is higher than the above upper limit, high output cannot beensured because the head used for recording is saturated.

According to the present invention, both a high H_(c) and an improvedH_(c) distribution can be imparted even to ferromagnetic metal particlesto which it has been difficult to impart a high coercive force and areduced content of high-coercive-force components, by directingattention to the crystallites which constitute the ferromagnetic metalparticles and controlling the crystallinity. It is thought that inconventional processes, attainment of a high H_(c) and improvement ofH_(c) distribution are insufficient because the starting material usedhas an insufficiently regulated shape and because the resultingferromagnetic metal particles are not regulated in the number and shapeof the crystallites constituting each ferromagnetic metal particle.

In the present invention, any desired method may be used for regulatingthe ferromagnetic metal particles without particular limitations.However, a preferred method is as follows.

A starting material having uniformity in particle size is subjected to asintering prevention treatment and then reduced, while regulating thenumber of metal (e.g., Fe) nuclei formed from a metal oxide (e.g., FeOor Fe₃ O₄) and further regulating the crystallinity. Examples of thestarting material include monodisperse goethite and monodispersehematite.

The starting material desirably has an average long axis length of from40 to 120 nm and an acicular ratio of from 3 to 10. If a startingmaterial having an average long axis length of smaller than 40 nm isused, H_(c) and σ_(s) cannot be regulated to values within therespective desired ranges. If a starting material having an average longaxis length of larger than 120 nm is used, it is difficult to heightenthe crystallinity, so that H_(c) cannot be heightened. If a startingmaterial having an acicular ratio of higher than 10 is used, it isdifficult to heighten the crystallinity. If the acicular ratio thereofis lower than 3, the ferromagnetic metal particles yielded therefromhave a reduced coercive force and are unusable for a high-densityrecording medium.

Means for regulating the ferromagnetic metal particles for use in thepresent invention are not particularly limited. Examples thereof includethe following methods (1) and (2).

(1) A specific elemental composition mainly for the inner part of theferromagnetic metal particles is specified.

In particular, in the case of ferromagnetic metal particles mainlycomprising Fe, specific minor elements which interact with Fe are used.Preferred examples of the minor elements include Ca, Co, Ni, and Cr.These minor elements are preferably added during the preparation ofgoethite or hematite and/or after the preparation thereof by means of asurface treatment.

(2) In the technique of producing ferromagnetic metal particles by thereduction of an oxide of a ferromagnetic metal element, conditions forpretreatments performed prior to the reduction, e.g., conditions for thedehydration or annealing of goethite, are selected and conditions forthe reduction, e.g., temperature, reducing gas, and reduction time, areselected.

For example, conditions for each step of the treatment ofgoethite-containing minor elements and obtained according to (1) aboveare as follows.

Dehydration may be conducted with a rotary type electric furnace in anitrogen atmosphere at generally 250° to 400° C., preferably 300° to400° C., for generally 0.5 to 2 hours, preferably 0.5 to 1 hour.Annealing may be conducted with a stationary reducing furnace in anitrogen atmosphere at generally 500° to 800° C., preferably 550° to700° C., for generally 1 to 5 hours, preferably 2 to 3 hours. Betweenthe dehydration and the annealing, a step of washing the resultinghematite with water to remove soluble alkali metals may be conducted.

Reduction may be conducted with a stationary reducing furnace in such amanner that the iron oxide is first reduced in a hydrogen atmosphere atgenerally 350° to 500° C., preferably 425° to 480° C., for generally0.25 to 1 hour, preferably 0.25 to 0.5 hours, and then heated in anitrogen atmosphere at generally 450° to 650° C., preferably 500° to600° C., for generally 0.5 to 3 hours, preferably 1 to 2 hours, andfurther then reduced in a pure hydrogen atmosphere at that temperaturefor 3 to 5 hours.

The completion of the reduction is determined by measuring the moisturecontent of the discharged gas with a dew-point hygrometer.

For producing the ferromagnetic metal particles described above, knownmethods may be used such as the methods described in JP-A-7-109122 andJP-A-6-340426.

Although the ferromagnetic metal particles are not particularly limitedin the ferromagnetic metal elements thereof, preferably, they compriseFe, Ni, or Co as their main component (at least 75%). An especiallypreferred element is Co, because it serves to enhance as and can form adense and thin oxide film. The content of Co atoms is preferably from 5to 45 atomic %, more preferably from 10 to 40 atomic %, based on theamount of Fe atoms. It is preferred that part of the necessary amount ofCo be incorporated into a starting material by doping and the remainderbe adhered to the surface of the doped starting material, before the Cois converted to an alloy through reduction.

The ferromagnetic metal particles for use in the present inventionpreferably contain up to 20 wt. % atoms of elements besides atoms of thegiven metals. Examples of the optional elements include Al, Si, S, Ti,V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Sr, W, Au, Pb, Bi, La, Ce,Pr, Nd, P, Mn, Zn, Sr, B, and Ca. These elements not only contribute toregulation of the shape of the starting material, but also are effectivein preventing sintering, accelerating reduction, and regulating theshape and surface roughness of the ferromagnetic metal particlesproduced through reduction.

The final reduction for completely reducing monodisperse goethite ormonodisperse hematite into the metal is conducted with pure hydrogen. Itis useful for heightening the crystallinity to conduct annealing in thecourse of the reduction at the stage of α-Fe₂ O₃. For the reduction ofα-Fe₂ O₃ to Fe₃ O₄ and FeO, various reducing gases may be used in steadof pure hydrogen. Since the presence of water during reductioninfluences the occurrence of sintering, it is necessary for heighteningthe crystallinity that the water resulting from reduction should berapidly removed from the system after the formation of metal nuclei froma metal oxide or during the subsequent formation of crystallites, orthat the reduction should be regulated so as to yield a reduced amountof water. This regulation of water amount can be accomplished bycontrolling the partial pressure or amount of a reducing gas.

An oxide film is formed on the surface of the ferromagnetic metalparticles may contain a small amount of a hydroxide or oxide. If the gasused for the gradual oxidation contains carbon dioxide, the carbondioxide is adsorbed onto basic sites present on the surface of theferromagnetic metal particles. The ferromagnetic metal particles whichhave undergone the gradual oxidation may contain such adsorbed carbondioxide.

In the conventional processes for producing magnetic metal particlesfrom goethite (α-FeOOH) or hematite (α-Fe₂ O₃) as a starting material,the particle contours attributable to the size and shape of the startingmaterial are large. Specifically, the conventional ferromagnetic metalparticles have an average long axis length of about 0.2 to 0.3 μm. Inthose conventional processes, the particle contours contractsimultaneously with reduction into the metal with elimination of oxygento give polycrystalline sparse metal particles as shown in FIG. 2. Inaddition, the crystallites of each metal particle are not uniform insize and shape, and the number of crystallites per particle is from 4 to10 or larger. In contrast, in the present invention, the size of theparticle contours attributable to the size and shape of a startingmaterial is reduced and the proportion of single-crystal particles inwhich, unlike the conventional polycrystalline state, each particle isconstituted of one crystallite as shown in FIG. 1 is increased as muchas possible in the ferromagnetic metal particles.

In the present invention, at least 30% of all ferromagnetic metalparticles each is constituted of one crystallite, and the proportion ofparticles each having more than one crystallite is below 70%. It isgenerally desired that the proportion of particles each having twocrystallites to all particles be 40 to 30%, that of particles eachhaving three crystallites to all particles be 10 to 0%, and that ofparticles each having four or more crystallites be zero.

In the present invention, a photograph of ferromagnetic metal particleswas taken with a high-resolution transmission electron microscope, andthe acicular ratio of the crystallites was determined from the averagelong axis length of the ferromagnetic metal particles and from thelattice images of the ferromagnetic metal particles. About 200 particleswere thus examined to determine the average long axis length of theferromagnetic metal particles. The acicular ratio of the crystalliteswas determined by tracing the contour of each crystallite image in thehigh-resolution electron photomicrograph with an image analyzer todetermine the long axis length and short axis length thereof andcalculating the value of (long axis length)/(short axis length).

Furthermore, the fine ferromagnetic metal particles may be treated with,for example, a dispersant, a lubricant, a surfactant, or an antistaticagent described below, before the dispersion. These treatments aredescribed in, for example, JP-B-44-14090, JP-B-45-18372, JP-B-47-22062,JP-B-47-22513, JP-B-46-28466, JP-B-46-38755, JP-B-47-4286,JP-B-47-12422, JP-B-47-17284, JP-B-47-18509, JP-B-47-18573,JP-B-39-10307, JP-B-48-39639, and U.S. Pat. Nos. 3,026,215, 3,031,341,3,100,194, 3,242,005, and 3,389,014.

The water content of the ferromagnetic metal particles is preferablyfrom 0.01 to 2% by weight, and is preferably optimized according to thekind of the binder, which will be described later.

The tap density of the ferromagnetic metal particles is preferably from0.2 to 0.8 g/ml. Ferromagnetic metal particles having a tap density ofhigher than 0.8 g/ml are undesirable since such particles are not evenlyoxidized in gradual oxidation, not only they are difficult to handlesafely, but also the tape obtained using the same undergoes a decreasein magnetization with the lapse of time. Ferromagnetic metal particleshaving a tap density of lower than 0.2 g/ml tend to be poorly dispersed.

The binder resin for use in the magnetic layer in the magnetic medium ofthe present invention may be a conventionally known thermoplastic resin,thermosetting resin, or reactive resin, or a mixture thereof.

The thermoplastic resin may be one having a glass transition temperatureof -100° to 150° C., a number-average molecular weight of 1,000 to200,000, preferably 10,000 to 100,000, and a degree of polymerization ofabout 50 to 1,000.

Examples of the thermoplastic resin include polymers or copolymerscontaining a structural unit derived from vinyl chloride, vinyl acetate,vinyl alcohol, maleic acid, acrylic acid, acrylate, vinylidene chloride,acrylonitrile, methacrylic acid, methacrylate, styrene, butadiene,ethylene, vinyl butyral, vinyl acetal, or vinyl ether; polyurethaneresins; and various rubber-type resins.

Examples of the thermosetting or reactive resin include phenolic resins,epoxy resins, thermosetting polyurethane resins, urea resins, melamineresins, alkyd resins, reactive acrylic resins, formaldehyde resins,silicone resins, epoxy-polyamide resins, mixtures of polyester resin andisocyanate prepolymer, mixtures of polyester polyol and polyisocyanate,and mixtures of polyurethane and polyisocyanate.

For obtaining further improved dispersibility of the ferromagneticparticles and durability of the magnetic layer, it is preferred to use,according to need, one or more of the above-enumerated binders whichhave incorporated therein through copolymerization or addition reaction,at least one polar group selected from --COOM, --SO₃ M, --OSO₃ M,--P═O(OM)₂, --O--P═O(OM)₂ (wherein M represents a hydrogen atom or analkali metal salt group), --OH, --NR₂, --N⁺ R₃ (R represents ahydrocarbon group), an epoxy group, --SH, and --CN. The amount of thepolar group(s) is generally from 10⁻¹ to 10⁻⁸ mol/g, preferably from10⁻² to 10⁻⁶ mol/g.

The amount of the binder resin for use in the magnetic recording mediumof the present invention is generally from 5 to 50% by weight,preferably from 10 to 30% by weight, based on the amount of theferromagnetic metal particles. When the vinyl chloride resin, thepolyurethane resin, and the polyurethane resin are preferably used incombination in an amount of 5 to 100% by weight, 2 to 50% by weight, and2 to 100% by weight.

The filling degree of the ferromagnetic metal particles are calculatedby the maximum saturation magnetization degree σs and Bm of theferromagnetic metal particles used (Bm/4πσs). In the present invention,the amount thereof is preferably 1.7 g/ml or more, more preferably 1.9g/ml or more, and most preferably 2.1 g/ml or more.

In using polyurethane in the present invention, the polyurethanepreferably has a glass transition temperature of -50° to 100° C., anelongation at break of 100 to 2,000%, a stress at break of 0.05 to 10kg/cm², and a yield point of 0.05 to 10 kg/cm².

Examples of the polyisocyanate for use in the present invention includeisocyanates such as tolylene diisocyanate, 4,4'-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylylene diisocyanate,naphthylene 1,5-diisocyanate, o-toluidine diisocyanate, isophoronediisocyanate, and triphenylmethane triisocyanate, products of thereactions of these isocyanates with polyalcohols, and polyisocyanatesformed through condensation of isocyanates. These isocyanates arecommercially available under the trade names of: Coronate L, CoronateHL, Coronate 2030, Coronate 2031, Millionate MR, and Millionate MTL(manufactured by Nippon Polyurethane Co., Ltd.); Takenate D-102,Takenate D-110N, Takenate D-200, and Takenate D-202 (manufactured byTakeda Chemical Industries, Ltd.); and Desmodule L, Desmodule Ill,Desmodule N, and Desmodule HL (manufactured by Sumitomo Bayer Co.,Ltd.). For each of the layers, these polyisocyanates may be used alone,or used in combination of two or more thereof, taking advantage of adifference in curing reactivity.

The magnetic layer of the present invention may contain additives whichhave various functions, such as lubricants, abrasives, dispersants,antistatic agents, plasticizers, and antimolds, if necessary.

Examples of lubricants for use in the magnetic layer of the presentinvention include silicone oils such as dialkylpolysiloxanes (each alkylhas from 1 to 5 carbon atoms), dialkoxypolysiloxanes (each alkoxy hasfrom 1 to 4 carbon atoms), monoalkylmonoalkoxypolysiloxanes (the alkyland the alkoxy have from 1 to 5 and from 1 to 4 carbon atoms,respectively), phenylpolysiloxane, and fluoroalkylpolysiloxanes (thealkyl has from 1 to 5 carbon atoms); electrically conductive fineparticles, e.g., graphite particles; inorganic powders, e.g., molybdenumdisulfide and tungsten disulfide; fine particles of plastics, e.g.,polyethylene, polypropylene, polyethylene-vinyl chloride copolymers, andpolytetrafluoroethylene; α-olefin polymers; saturated fatty acids whichare solid at ordinary temperature (having from 10 to 22 carbon atoms);unsaturated aliphatic hydrocarbons which are liquid at ordinarytemperature (compounds having an n-olefin double bond bonded to aterminal carbon atom, and having about 20 carbon atoms); fatty acidesters formed from monobasic fatty acids having from 12 to 20 carbonatoms and monohydric alcohols having from 3 to 12 carbon atoms; andfluorocarbons.

Of the above-enumerated lubricants, the saturated fatty acids and thefatty acid esters are preferred, with a combination of both beingpreferred. Examples of alcohols usable as starting materials for thefatty acid esters include monohydric alcohols such as ethanol, butanol,phenol, benzyl alcohol, 2-methylbutyl alcohol, 2-hexyldecyl alcohol,propylene glycol monobutyl ether, ethylene glycol monobutyl ether,dipropylene glycol monobutyl ether, diethylene glycol monobutyl ether,and s-butyl alcohol; and polyhydric alcohols such as ethylene glycol,diethylene glycol, neopentyl glycol, glycerol, and sorbitan derivatives.Examples of fatty acids usable as starting materials for the fatty acidesters include aliphatic carboxylic acids such as acetic acid, propionicacid, octanoic acid, 2-ethylhexanoic acid, lauric acid, myristic acid,stearic acid, palmitic acid, behenic acid, arachic acid, oleic acid,linoleic acid, linolenic acid, elaidic acid, and palmitoleic acid, andmixtures of these acids.

Specific examples of the fatty acid esters include butyl stearate,s-butyl stearate, isopropyl stearate, butyl oleate, amyl stearate,3-methylbutyl stearate, 2-ethylhexyl stearate, 2-hexyldecyl stearate,butyl palmitate, 2-ethylhexyl myristate, butyl stearate/butyl palmitatemixtures, butoxyethyl stearate, 2-butoxy-1-propyl stearate, dipropyleneglycol monobutyl ether stearate, diethylene glycol dipalmitate,hexamethylenediol dimyristate, and glycerol oleate.

In order to diminish the hydrolysis of a fatty acid ester which oftenoccurs during use of the magnetic recording medium in a high-humidityatmosphere, a technique is used in which branched/linear isomers,cis/trans isomers, other isomers, and branching sites are taken inaccount when selecting a fatty acid and an alcohol used as startingmaterials for the ester.

These lubricants may be added in an amount of 0.2 to 20 parts by weightper 100 parts by weight of the binder.

Other usable lubricants include silicone oils, graphite, molybdenumdisulfide, boron nitride, fluorinated graphite, fluorinated alcohols,polyolefins, polyglycols, alkyl phosphates, and tungsten disulfide.

Examples of abrasives for use in the magnetic layer of the presentinvention include generally employed abrasive materials such asα-alumina, γ-alumina, fused alumina, corundum, artificial corundum,silicon carbide, chromium oxide (Cr₂ O₃), diamond, artificial diamond,garnet, emery (main components; corundum and magnetite), and α-Fe₂ O₃.These abrasive materials have a Mohs' hardness of 6 or more. Specificexamples thereof include AKP-10, AKP-12, AKP-15, AKP-20, AKP-30, AKP-50,AKP-1520, AKP-1500, HIT-50, HIT60A, HIT70, HIT80, and HIT-100(manufactured by Sumitomo Chemical Co., Ltd.); G5, G7, S-1, and ChromiumOxide K (manufactured by Nippon Chemical Industrial Co., Ltd.); UB40B(manufactured by Uemura Kogyo K.K.); WA8000 and WA10000 (manufactured byFujimi Kenmazai Kogyo Co., Ltd.); and TF100, TF140, and TF180(manufactured by Toda Kogyo Co., Ltd.). Effective are abrasives havingan average particle diameter of generally 0.05 to 3 μm, preferably from0.05 to 1.0 μm.

The total addition amount of these abrasives is generally from 1 to 20parts by weight, preferably from 1 to 15 parts by weight, per 100 partsby weight of the magnetic particle. If the amount of abrasives issmaller than 1 part by weight, sufficient durability cannot be obtained.If the amount thereof is larger than 20 parts by weight, surfaceproperties and filling degree are impaired. These abrasives may bedispersed into a binder, before being added to a magnetic coatingcomposition.

Electrically conductive particles may be incorporated as an antistaticagent, besides the nonmagnetic particles described above, into themagnetic layer of the magnetic recording medium of the presentinvention. However, from the standpoint of forming an uppermost layerhaving a saturation flux density heightened to the highest level, it ispreferred to incorporate most of the electrically conductive particlesinto a layer other than the uppermost layer to minimize the amountthereof incorporated in the uppermost layer. Carbon black is especiallypreferably incorporated as an antistatic agent from the standpoint ofreducing the surface electrical resistance of the whole medium. Examplesof the carbon black for use in the present invention include furnaceblack for rubbers, thermal black for rubbers, coloring black, conductivecarbon blacks, and acetylene black. The carbon black preferably has aspecific surface area of 5 to 500 m² /g, a DBP absorption amount of 10to 1,500 ml/100 g, a particle diameter of 5 to 300 mμ, a pH of 2 to 10,a water content of 0.1 to 10%, and a tap density of 0.1 to 1 g/ml.Specific examples of the carbon black include BLACKPEARLS 2000, 1300,1000, 900, 800, 700, and VULCAN XC-72 (manufactured by Cabot Co., Ltd.);#80, #60, #55, #50, and #35 (manufactured by Asahi Carbon Co., Ltd.);#3950B, #3250B, #2700, #2650, #2600, #2400B, #2300, #900, #1000, #95,#30, #40, #10B, MA230, MA220, and MA77 (manufactured by MitsubishiChemical Corp.); CONDUCTEX SC, RAVEN 150, 50, 40, and 15 (manufacturedby Columbian Carbon Co.); and Ketjen Black EC, Ketjen Black ECDJ-500,and Ketjen Black ECDJ-600 (manufactured by Lion Akzo Co., Ltd.). Thesecarbon blacks may be surface-treated with a dispersant or other agent,oxidized, or grafted with a resin before use. The carbon black whosesurface has been partly graphitized may also be used. Furthermore,before being added to a magnetic coating composition, the carbon blackmay be dispersed into a binder. In using the carbon black in themagnetic layer, the amount thereof is preferably from 0.1 to 30% byweight based on the amount of the magnetic particle. In forming anonmagnetic layer, the carbon black is preferably incorporated thereinin an amount of 3 to 20% by weight based on the amount of the inorganicnonmagnetic particles.

In general, the carbon black functions as an antistatic agent, andfurther serves to reduce the coefficient of friction, to impart a lightscreen, and to improve film strength. These effects are produced todifferent degrees depending on the kind of carbon black used. Therefore,it is, of course, possible in the present invention to change the kind,amount, and combination of carbon blacks according to purposes on thebasis of the above-described properties including particle size, oilabsorption amount, electrical conductivity, and pH. With respect tocarbon blacks usable in the present invention, reference may be made to,for example, Carbon Black Binran (Carbon Black Handbook), edited byCarbon Black Association.

In forming a nonmagnetic layer between the magnetic layer and thenonmagnetic support in the magnetic recording medium of the presentinvention, the nonmagnetic layer (hereinafter often referred to also asa "lower layer") is a layer consisting mainly of inorganic nonmagneticparticles dispersed in a binder resin. Various materials can be used asthe inorganic nonmagnetic particles for the nonmagnetic layer. Examplesthereof include α-alumina having an alpha-conversion of 90% or more,β-alumina, γ-alumina, silicon carbide, chromium oxide, cerium oxide,α-iron oxide, corundum, silicon nitride, titanium carbide, titaniumoxide, silicon dioxide, boron nitride, zinc oxide, calcium carbonate,calcium sulfate, and barium sulfate. These may be used alone or incombination. These inorganic nonmagnetic particles preferably have aparticle size of 0.01 to 2 μm. If needed, inorganic nonmagneticparticles having different particle sizes may be used in combination, ora single inorganic nonmagnetic particles having a widened particlediameter distribution may be used so as to produce the same effect. Theinorganic nonmagnetic particles to be used may have been surface-treatedso as to enhance interaction with the binder resin used to therebyimprove dispersibility. The surface treatment may be performed witheither an inorganic substance, e.g., silica, alumina, or silica-alumina,or a coupling agent. The inorganic nonmagnetic particles preferably havea tap density of 0.3 to 2 g/ml, a water content of 0.1 to 5% by weight,a pH of 2 to 11, and a specific surface area of 5 to 100 m² /g. Theinorganic nonmagnetic particles may have any particle shape selectedfrom the acicular, spherical, cubical, and tabular forms. Specificexamples of the inorganic nonmagnetic particles for use in the presentinvention include AKP-20, AKP-30, AKP-50, and HIT-50 (manufactured bySumitomo Chemical Co., Ltd.); G5, G7, and S-1 (manufactured by NipponChemical industrial Co., Ltd.); TF-100, TF-120, and TF-140 (manufacturedby Toda Kogyo Co., Ltd.); TTO 55 Series and ET300W (manufactured byIshihara Sangyo Kaisha, Ltd.); STT30 (manufactured by Titan Kogyo CO.,LTD.); and acicular hematite particles used as an intermediate for amagnetic iron oxide or an intermediate for producing ferromagnetic metalparticles by the iron oxide reduction method.

The layer constitution of the magnetic recording medium of the presentinvention is not particularly limited, as long as the medium basicallycomprises at least a nonmagnetic support and the above-describedmagnetic layer formed thereover or further has the nonmagnetic layerdescribed above. A magnetic or nonmagnetic layer having anothercomposition may be formed. For example, in the case of forming aferromagnetic layer in place of the above-described nonmagnetic layer,various ferromagnetic particles can be used therefor, such as, e.g., aferromagnetic iron oxide, a cobalt-modified ferromagnetic iron oxide,CrO₂, a hexagonal ferrite, and other ferromagnetic metals. Theseferromagnetic materials are dispersed in a resin to form theferromagnetic layer. This ferromagnetic layer is also referred to as alower layer.

Forming two or more coating layers on a nonmagnetic support is effectivein producing a magnetic recording medium for high-density recording. Asimultaneous coating technique is especially superior since an ultrathinmagnetic layer can be formed. Specific examples of wet-on-wet coating asthe simultaneous coating techniques include the following methods.

1. A lower layer is first applied with a coating apparatus commonly usedfor magnetic coating composition, e.g., a gravure coating, roll coating,blade coating, or extrusion coating apparatus, and an upper layer isthen applied, while the lower layer is in a wet state, by means of asupport-pressing extrusion coater such as those disclosed inJP-B-1-46186, JP-A-60-238179, and JP-A-2-265672.

2. An upper layer and a lower layer are applied almost simultaneouslyusing a single coating head having therein two slits for passing coatingcompositions, such as those disclosed in JP-A-63-88080, JP-A-2-17971,and JP-A-2-265672.

3. An upper layer and a lower layer are applied almost simultaneouslywith an extrusion coater equipped with a back-up roll, such as thatdisclosed in JP-A-2-174965.

In wet-on-wet coating, it is preferred that the flow characteristics ofthe coating composition for magnetic-layer formation be akin as much aspossible to those of the coating composition for nonmagnetic-layerformation, because the interface between the magnetic and nonmagneticcoating layers formed therefrom has no disturbance and, as a result, themagnetic layer obtained can have uniformity in thickness with diminishedthickness fluctuations. Since the flow characteristics of a coatingcomposition largely depend on the combination of the particles and thebinder resin contained in the coating composition, special care shouldbe taken in selecting nonmagnetic particles for use in the nonmagneticlayer.

The nonmagnetic support used in the magnetic recording medium of thepresent invention has a thickness of generally 1 to 100 μm, andpreferably 3 to 20 μm (as a tape) or 40 to 80 μm (as a flexible disk).The nonmagnetic layer has a thickness of generally 0.5 to 10 μm, andpreferably 0.5 to 3 μm. When a magnetic layer is formed on a nonmagneticlayer, the thickness of the magnetic layer is generally 0.05 to 3.0 μm,and preferably 0.05 to 1.0 μm.

Layers other than the magnetic and nonmagnetic layers described abovecan be formed according to purposes. For example, an undercoat layer maybe formed between the nonmagnetic support and the lower layer in orderto improve adhesion. This undercoat layer has a thickness of generally0.01 to 2 μm, and preferably 0.05 to 0.5 μm. A back coat layer may beformed on the surface of the nonmagnetic support opposite to themagnetic layer. This back coat layer has a thickness of generally 0.1 to2 μm, and preferably 0.3 to 1.0 μm. These undercoat and back coat layersmay be known layers. In a disk-form magnetic recording medium, theabove-described layers may be formed on one or both sides.

The nonmagnetic support for use in the present invention is notparticularly limited, and ordinarily employed nonmagnetic supports maybe used in the present invention. Examples of materials used forconstituting nonmagnetic supports include films of various syntheticresins such as poly(ethylene terephthalate), polyethylene,polypropylene, polycarbonates, poly(ethylene naphthalate), polyamides,poly(amide-imide)s, polyimides, polysulfones, and polyethersulfones andmetal foils such as aluminum foil and stainless-steel foil.

In order to attain the objects of the present invention, it is preferredto use a nonmagnetic support having a center line average surfaceroughness (Ra: cutoff value: 0.25 mm) of 0.03 μm or less, morepreferably 0.02 μm or less, and particularly preferably 0.01 μm or less.In addition that the nonmagnetic recording medium of the presentinvention has a small center line average surface roughness, thenonmagnetic support is preferably free from projections as large as 1 μmor more. The state of the surface roughness of the support can be freelycontrolled by changing the size and amount of a filler which isincorporated into the support if needed. Examples of the filler includeoxides or carbonates of Ca, Si, and Ti and fine organic powders such asacrylic powder. The nonmagnetic support for use in the present inventionpreferably has an F-5 value in the web running direction of 5 to 50kg/mm² and an F-5 value in the web width direction of 3 to 30 kg/mm².Although the F-5 value in the web longitudinal direction is generallyhigher than that in the web width direction, this does not apply in thecase where the width direction strength, in particular, should beenhanced.

The degrees of thermal shrinkage of the support in the web runningdirection and in the web width direction are preferably 3% or less, morepreferably 1.5% or less, under conditions at 100° C. for 30 minutes, andare preferably 1% or less, more preferably 0.5% or less, underconditions at 80° C. for 30 minutes. The strength at break thereof ispreferably from 5 to 100 kg/mm², and the modules thereof is preferablyfrom 100 to 2,000 kg/mm².

Examples of organic solvents for use in the present invention includeketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone,diisobutyl ketone, cyclohexanone, isophorone, and tetrahydrofuran;alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol,isopropyl alcohol, and methylcyclohexanol; esters such as methylacetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyllactate, and glycol acetate; glycol ethers such as glycol dimethylethers, glycol monoethyl ethers, and dioxane; aromatic hydrocarbons suchas benzene, toluene, xylene, cresol, and chlorobenzene; chlorinatedhydrocarbons such as methylene chloride, ethylene chloride, carbontetrachloride, chloroform, ethylene chlorohydrin, and dichlorobenzene;and other compounds such as N,N-dimethylformamide and hexane. Thesesolvents may be used in arbitrary proportions. These organic solventsneed not be 100% pure, and may contain impurities, such as isomers,unreacted raw materials, by-products, decomposition products, oxidationproducts, and water, besides the main components. The content of theseimpurities is preferably 30% or less, more preferably 10% or less. Inthe present invention, the organic solvents used in the magnetic layerand the nonmagnetic layer may be different in kind and amount, ifneeded. For example, it is possible to use a highly volatile solvent forforming a nonmagnetic layer having improved surface properties, to use asolvent with a high surface tension (e.g., cyclohexane or dioxane) forimproving the stability of the coating for forming a nonmagnetic layer,or to use a solvent with a high solubility parameter for forming amagnetic layer for increasing filling degree. However, methods of usingorganic solvents are, of course, not limited to these examples.

The magnetic recording medium of the present invention is obtained by aprocess comprising kneading the above-described ferromagnetic metalparticles and binder resin together with an organic solvent if necessaryfurther with other optional additives, applying the resulting magneticcoating composition on a nonmagnetic support, and drying the coatingcomposition. Prior to the drying step, orientation may be conducted ifneeded.

The process for preparing a magnetic coating composition to be used forproducing the magnetic recording medium of the present inventioncomprises at least a kneading step and a dispersing step, and mayfurther comprise a mixing step that may be conducted, if necessary,before and after the two steps. Each step may include two or morestages. Each of the materials for use in the present invention,including the ferromagnetic metal particles, binder, carbon black,abrasive material, antistatic agent, lubricant, and solvent, may beadded in any step either at the beginning of or during the step.Furthermore, each ingredient may be added portion-wise in two or moresteps. For example, a polyurethane may be added portion-wise in each ofthe kneading step, the dispersing step, and the mixing step forviscosity adjustment after the dispersion.

For kneading and dispersion for preparing a magnetic coatingcomposition, any of various kneading machines may be used. Examplesthereof include a two-roll mill, three-roll mill, ball mill, pebblemill, tron mill, sand grinder, Szegvari, attritor, high-speed impellerdispersing machine, high-speed stone mill, high-speed impact mill,disper, kneader, high-speed mixer, homogenizer, and ultrasonicdispersing machine.

Conventionally known manufacturing techniques can, of course, be used aspart of the process to attain the object of the present invention. Itis, however, preferred to use a kneading machine having high kneadingpower, such as a continuous kneader or pressure kneader, in the kneadingstep. In the case of using a continuous kneader or pressure kneader, theferromagnetic metal particles are kneaded together with all or part(preferably at least 30%) of the binder, the binder amount being in therange of 15 to 500 parts by weight per 100 parts by weight of theferromagnetic metal particles. Details of this kneading treatment aredescribed in JP-A-1-106338 and JP-A-64-79274. The magnetic recordingmedium of the present invention can be efficiently produced by employinga technique of simultaneous multiple coating as disclosed inJP-A-62-212933.

The residual solvent content in the magnetic layer in the magneticrecording medium of the present invention is preferably 100 mg/m² orless, more preferably 10 mg/m² or less. It is preferred that theresidual solvent content in the magnetic layer be lower than that in thenonmagnetic layer.

The void content in the lower layer and that in the upper layer are eachpreferably 30% by volume or less, and more preferably 10% by volume orless. The nonmagnetic layer preferably has a higher void content thanthe magnetic layer, but may have a lower void content as long as itsvoid content is 5% by volume or more.

The magnetic recording medium of the present invention, which may have alower layer and an upper layer, can be made to have a difference inphysical property between the upper and lower layers according topurposes, as can be easily presumed. For example, the upper layer ismade to have a heightened modulus to improve running durability and, atthe same time, the lower layer is made to have a lower modulus than themagnetic layer to improve the head touching of the magnetic recordingmedium.

The magnetic coating layer thus formed on a support is subjected, ifnecessary, to an orientation treatment for orienting the ferromagneticmetal particles contained in the layer, and then dried. Asurface-smoothing treatment may be performed if needed, and theresulting structure is cut into a desired shape. Thus, the magneticrecording medium of the present invention is produced. Theabove-described composition for upper-layer formation and the optionalcomposition for lower-layer formation each is dispersed along with asolvent. The coating compositions thus obtained are applied to anonmagnetic support, and orientation and drying are conducted to obtaina magnetic recording medium.

The magnetic layer preferably has a modulus at 0.5% elongation of 100 to2,000 kg/mm² in each of the machine and transverse directions and astrength at break of 1 to 30 kg/cm². The magnetic recording mediumpreferably has a modulus of 100 to 1,500 kg/mm² in each of the machineand transverse directions and a residual elongation of 0.5% or less. Thedegree of thermal shrinkage of the magnetic recording medium at anytemperature of not higher than 100° C. is preferably 1% or less, morepreferably 0.5% or less, and most preferably 0.1% or less.

The magnetic recording medium of the present invention may be a tape forvideo, audio, or another use, or may be a floppy disk or magnetic diskfor data recording. The recording medium of this invention isparticularly effective when used as a digital recording medium in whichthe occurrence of signal dropouts should be avoided by all means. Byemploying a constitution in which the lower layer is a nonmagnetic layerand the uppermost layer has a thickness of 1 μm or less, alarge-capacity magnetic recording medium can be obtained which has highelectro-magnetic characteristics and excellent overwritingcharacteristics and is suitable for high-density recording.

EXAMPLE

The present invention will be explained in detail by reference to thefollowing Examples and Comparative Examples, but the invention shouldnot be construed as being limited thereto. All percents, parts, ratiosand the like are by weight unless otherwise indicated.

PRODUCTION EXAMPLE 1 Production Examples 1-1 to 1-4

(Production of Ferromagnetic Metal Particles)

To a mixture of 35 liters of 1.7 mol/l sodium carbonate and 15 liters of2.0 mol/l sodium hydroxide placed in a 150-liter tank equipped with astirrer was added 0.6 liters of 0.5 mol/l aqueous sodium phosphatesolution. While nitrogen was continuously bubbled thereinto at a liquidtemperature of 20° C., 40 liters of an aqueous solution of ferroussulfate and cobalt sulfate (Fe²⁺ concentration, 1.35 mol/l; Coconcentration, 0.15 mol/l) having a temperature of 20° C. which wasprepared in another tank with nitrogen bubbling was added thereto. Afterthe resulting mixture was stirred for 10 minutes, the temperature of thesuspension was adjusted to 25° C. to form a precipitate containingferrous iron as the main component. Air was introduced in place ofnitrogen to oxidize the precipitate to thereby yield nuclear goethitecrystals. At the time when the Fe² + concentration of the suspensionreached 0.75 mol/l, the oxidation with air was stopped. Nitrogen wasintroduced in place of air, and the suspension was heated to 40° C. andmaintained at this temperature for 2 hours, following which 1.5 litersof 1.1 mol/l aqueous sodium aluminate solution was added. Thereafter,air was introduced in place of nitrogen to further conduct an oxidationreaction to thereby yield spindle-shaped particles of a solid solutionof Al in goethite. The particles obtained were taken out by filtrationand washed with water. Part of the particles were dried and photographedwith a transmission electron microscope to determine the averageparticle diameter thereof. As a result, the particles were found to havean average long axis length of 0.10 μm and an average acicular ratio of7. Further, the specific surface area of the particles was measuredafter dehydration by 30-minute heating at 120° C. in nitrogen, and wasfound to be 110 m² /g.

The goethite obtained was dispersed into water to give a 2% slurry. Anaqueous solution of cobalt sulfate and/or an aqueous solution ofmagnesium chloride was added to the slurry with stirring so as to resultin the Co and/or Mg addition amount (amount of the atoms (%) based onthe iron atom amount) shown in Table 1. This slurry was neutralized withammonia water to deposit a cobalt compound and/or a magnesium compoundon the surface of the particles. The slurry was filtered, and theparticles taken out were dispersed into water to give a 2% slurry again.Aqueous sodium aluminate solution was added thereto (the Al atom (%)based on the iron atom amount is shown in Table 1), and the resultingmixture was stirred for 20 minutes. Thereafter, dilute sulfuric acid wasadded to neutralize the slurry. The particles were taken out byfiltration, washed with water, and dispersed into water to give a 2%slurry. Aqueous yttrium nitrate solution was added (the Y atom (%) basedon the iron atom amount is shown in Table 1), and the pH of the slurrywas adjusted to 8.5 with ammonia water. The particles were taken out byfiltration, washed with water, and dispersed into water to give a 5%slurry, which was then heated at 150° C. for 1 hour. Thereafter, theparticles were taken out by filtration and washed with water. Theresulting cake was compacted with a compactor and then dried to obtainspindle-shaped goethite particles which had undergone a sinteringprevention treatment.

The spindle-shaped goethite obtained was placed in a stationary reducingfurnace, and heated in nitrogen at 350° C. for 30 minutes fordehydration and then at 650° C. for 2 hours to enhance the crystallineproperty of the hematite. The temperature was lowered to 400° C., and ahydrogen/CO=20/80 mixed gas was introduced in place of nitrogen toconduct reduction for 1 hour. Thereafter, nitrogen was introduced inplace of the mixed gas, and pure hydrogen was then introduced in placeof nitrogen to conduct reduction for 5 hours. Nitrogen was introduced inplace of pure hydrogen to cool the contents to room temperature, and anair/nitrogen mixture regulated to have an oxygen concentration of 0.2%was introduced to conduct gradual oxidation at 50° C. or lower whilemonitoring the temperature of the metal powder. After heat generationended, the oxygen concentration was adjusted to 1% to continue gradualoxidation for 10 hours. Thereafter, a vapor of distilled water wasintroduced together with air in such an amount as to result in a wateramount of 1% based on the amount of the metal particle to humidify andstabilize the particle.

Magnetic characteristics of the thus-obtained ferromagnetic metalparticles were determined with a vibrating sample magnetometer(manufactured by Toei Kogyo K.K., Japan) at an external magnetic fieldof 10 kOe. Each ferromagnetic metal particles obtained was photographedwith a high-resolution transmission electron microscope to determine theaverage long axis length, average acicular ratio, and crystallinity ofthe ferromagnetic metal particles and the average acicular ratio of thecrystallites. Further, the specific surface area (S_(BET)) of eachpowder was measured with Quantasorb (manufactured by Quantachrome Inc.)after 30-minute dehydration at 250° C. in nitrogen. The results obtainedare shown in Table 1.

PRODUCTION EXAMPLE 2 Production Examples 2-1 to 2-3

Using the elements shown in Table 1, spindle-shaped goethite particleswhich had undergone a sintering prevention treatment were obtainedthrough the same steps as in Production Example 1. The goethiteparticles were placed in a stationary reducing furnace, and dehydratedat 350° C. for 60 minutes in nitrogen. The temperature was elevated to450° C., and pure hydrogen was introduced in place of nitrogen toconduct reduction for 6 hours. The subsequent treatments were carriedout in the same manner as in Production Example 1. The ferromagneticmetal particles thus obtained were evaluated in the same manner as inProduction Example 1. The results obtained are shown in Table 1.

PRODUCTION EXAMPLE 3 Production Examples 3-1 and 3-2

Goethite was produced in the same manner as in Production Example 1,except the following. After nuclear goethite crystals were formed at 25°C. and at the time when the Fe²⁺ concentration of the suspension reached0.75 mol/l, oxidation with air was stopped and nitrogen was introducedin place of air. Sodium aluminate was added, and the suspension washeated to 50° C. to conduct oxidation with air. The goethite thusyielded had an average long axis length of 0.20 μm, an average acicularratio of 14, and a specific surface area of 130 m² /g. Subsequently, thegoethite was treated using the elements shown in Table 2 in the samemanner as in Production Example 1, and then treated with theantisintering agents. This goethite was taken out by filtration, washedwith water, compacted with a compactor, and then dried. Thereafter, thematerial obtained was reduced and subjected to gradual oxidation in thesame manner as in Production Example 1. The ferromagnetic metalparticles thus obtained were evaluated in the same manner as inProduction Example 1. The results obtained are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                             Ferromagnetic Metal Particle                              Element                                                                            Antisinter-    Average                                                                            Average    Average                              Production                                                                         added                                                                              ing Agent                                                                          H.sub.c                                                                          σ.sub.s                                                                     S.sub.BET                                                                        Long Axis                                                                          Acicular   Acicular Ratio                       Example                                                                            Mg                                                                              Co Al Y (Oe)                                                                             emu/g                                                                             m.sup.2 /g                                                                       Length                                                                             Ratio                                                                              Crystallinity                                                                       of Crystallites                      __________________________________________________________________________    1-1  0.5                                                                             0.0                                                                              8.0                                                                              6.0                                                                             1900                                                                             136.0                                                                             55.0                                                                             70   4.5  37    3.0                                  1-2  0.5                                                                             10 8.0                                                                              7.0                                                                             2120                                                                             140.5                                                                             53.5                                                                             68   4.0  45    2.8                                  1-3  1.0                                                                             20 5.0                                                                              8.0                                                                             2340                                                                             148.5                                                                             47.7                                                                             63   4.0  50    2.9                                  1-4  0.0                                                                             15 5.0                                                                              8.0                                                                             2210                                                                             143.4                                                                             53.7                                                                             64   4.3  42    2.7                                  2-1  0.5                                                                             0.0                                                                              8.0                                                                              6.0                                                                             1630                                                                             132.3                                                                             57.8                                                                             75   6.8  22    3.4                                  2-2  0.5                                                                             10 8.0                                                                              7.0                                                                             1800                                                                             135.5                                                                             56.3                                                                             73   6.5  26    3.2                                  2-3  1.0                                                                             20 5.0                                                                              8.0                                                                             2015                                                                             137.5                                                                             51.7                                                                             68   5.4  24    3.0                                  3-1  0.5                                                                             0.0                                                                              8.0                                                                              6.0                                                                             1750                                                                             134.3                                                                             57.0                                                                             125  7.7  20    3.6                                  3-2  1.0                                                                             20 8.0                                                                              7.0                                                                             1980                                                                             138.5                                                                             55.4                                                                             110  7.5  24    3.5                                  __________________________________________________________________________

EXAMPLE 1

Production of Magnetic Recording Media

Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-5

The composition for magnetic-layer formation and the composition fornonmagnetic-layer formation both shown below were prepared in order toproduce multilayered magnetic tapes respectively using the ferromagneticmetal particles obtained in Production Examples 1-1 to 1-5, 2-1 to 2-3,and 3-1 and 3-2. In the following formulations, all "parts" are byweight.

    ______________________________________                                        (Composition for Magnetic-Layer Formation)                                    Ferromagnetic metal particle                                                                            100    parts                                        (shown in Table 2)                                                            Binder resin                                                                  Vinyl chloride copolymer  13     parts                                        (--SO.sub.3 Na group content, 1 × 10.sup.-4 eq/g;                       degree of polymerization, 300)                                                Polyester polyurethane resin                                                                            5      parts                                        (neopentyl glycol/caprolactonepolyol/MDI =                                    0.9/2.6/1 (by mol)                                                            --SO.sub.3 Na group content, 1 × 10.sup.-4 eq/g)                        α-Alumina           5.0    parts                                        (average particle diameter, 0.13 μm)                                       Carbon black              1.0    part                                         (average particle diameter, 40 nm)                                            Butyl stearate            1      part                                         Stearic acid              2      parts                                        Methyl ethyl ketone/cyclohexanone                                                                       200    parts                                        (1:1) mixed solvent                                                           (Composition for Nonmagnetic-Layer Formation)                                 Acicular hematite         80     parts                                        (BET specific surface area, 55 m.sup.2 /g;                                    average long axis length, 0.12 μm;                                         acicular ratio, 8; pH, 8.8; aluminum                                          treatment: Al atom amount, 6.5 atom %                                         based on Fe atoms)                                                            Carbon black              20     parts                                        (average primary particle diameter, 17 nm;                                    DBP absorption amount, 80 ml/100 g;                                           BET specific surface area, 240 m.sup.2 /g;                                    pH, 7.5)                                                                      Binder resin                                                                  Vinyl chloride copolymer  12     parts                                        (--SO.sub.3 Na group content, 1 × 10.sup.-4 eq/g;                       degree of polymerization, 300)                                                Polyester polyurethane resin                                                                            5      parts                                        (neopentyl glycol/caprolactonepolyol/MDI =                                    0.9/2.6/1 (by mol);                                                           --SO.sub.3 Na group content, 1 × 10.sup.- 4 eq/g)                       Butyl stearate            1      part                                         Stearic acid              2.5    parts                                        Methyl ethyl ketone/cyclohexanone                                                                       200    parts                                        (1:1) mixed solvent                                                           ______________________________________                                    

With respect to each of the composition for magnetic-layer formation andthe composition for nonmagnetic-layer formation, the ingredients werekneaded with a kneader and then dispersed with a sand grinder. To theresulting dispersions was added a polyisocyanate in an amount of 5 partsfor the coating composition for nonmagnetic-layer formation and in anamount of 6 parts for the coating composition for magnetic-layerformation. To each dispersion was further added 20 parts of a methylethyl ketone/cyclo-hexanone (1:1) mixed solvent. These dispersions werefiltered through a filter having an average opening diameter of 1 μm toprepare a coating composition for nonmagnetic-layer formation and acoating composition for magnetic-layer formation.

The two coating composition obtained were applied on a 7 μm-thickpoly(ethylene terephthalate) support by wet-on-wet simultaneousmulti-layered coating as follows. The coating composition fornonmagnetic-layer formation was applied first at a dry thickness of 1.5μm, and the coating composition for magnetic-layer formation was appliedimmediately thereafter on the still wet nonmagnetic layer coating insuch an amount as to obtain a 0.15 μm-thick magnetic layer. While thetwo coating layers were still in a wet state, longitudinal orientationwas performed by passing the coated support through an orientationapparatus. In this orientation, the coated support was passed by a rareearth magnet (surface flux density, 5,000 G) and then passed through asolenoid magnet (flux density, 5,000 G), and was dried within thesolenoid to such a degree that the oriented state was maintained. Themagnetic layer was further dried, before the coated support was woundup. Thereafter, calendering was conducted with a 7-roll calendercomprising metal rolls at a roll temperature of 90° C. to obtain amagnetic recording medium as a web. The web was slit into an 8-mm width.Thus, 8-mm video tape samples were produced. The samples were examinedfor magnetic characteristics with a vibrating sample magnetometer andfor surface roughness. The samples were further examined with a drumtester for 1/2 Tb output, C/N, and overwriting characteristics. Theresults obtained are shown in Table 2. As a standard for the examinationof electromagnetic characteristics, Super DC Tape (manufactured by FujiPhoto Film Co., Ltd.) was used.

Overwriting characteristics were examined using a drum tester by thefollowing method. At a relative speed of a TSS head (head gap, 0.2 μm;track width, 14 μm; saturation flux density, 1.1 T) of 10.2 m/sec,signals of 1/90 Tb (λ=22.5 μm) were recorded at the optimum recordingcurrent determined from the input/output characteristics of signals of1/2 Tb (λ=0.5 μm), and signals of 1/2 Tb were then written thereover.From the degree of erasion of the recorded 1/90 Tb signals, theoverwriting characteristics were determined.

Magnetic characteristics were determined in the direction parallel tothe orientation direction with a vibrating sample magnetometer(manufactured by Toei Kogyo K.K.) at an intensity of external magneticfield of 5 kOe. SQ means squareness ratio. The content of high-H_(c)components was determined as follows. A magnetic recording medium samplewas set on the vibrating sample magnetometer (manufactured by Toei KogyoK.K.), in such a manner that the orientation direction for the samplewas the same as the direction of magnetic field. A magnetic field of -10kOe was applied to bring the sample into DC saturation, and the magneticfield was then returned to zero to measure the residual magnetization(-M_(rmax)). A magnetic field of 3,000 Oe was applied in the oppositedirection, and the magnetic field was then returned to zero to measurethe residual magnetization M_(r). Thereafter, a magnetic field of 10 kOewas applied to bring the sample into DC saturation in that oppositedirection, and the magnetic field was then returned to zero to measurethe residual magnetization M_(rmax). The content of high-H_(c)components was calculated from the thus-obtained residual magnetizationvalues using the following equation.

    High-H.sub.c component (%)=100×(M.sub.rmax -M.sub.r)/(M.sub.rmax -(-M.sub.rmax))

Although the magnetic field applied in that opposite direction can haveany desired intensity, an intensity of 3,000 Oe was employed herein(r3000) from the standpoint of detection sensitivity. The high-H_(c)components are components which undergo reversal of magnetization atthat magnetic field intensity or higher. Since r3000 necessarilyincreases with increasing value of average H_(c), it is necessary tostandardize r3000 with H_(c) for comparison among samples. The resultsobtained are shown in Table 3 in terms of (r3000/H_(c))×100.

Each sample was further examined with a torque magnetometer (Type TRT-2,manufactured by Toei Kogyo K.K.) at various magnetic field intensitiesto determine the magnetic torque curve for the film plane. From theresults obtained, the rotational hysteresis loss integral was calculatedand is shown to utilize the same as a measure of the mode of reversal ofmagnetization.

Surface roughness was measured by examining a sample area 250 μm squarewith light-interference three-dimensional roughness meter "TOPO-3D,"manufactured by WYKO Inc. (Ariz., U.S.A.). In calculation from the foundvalues, corrections such as slope correction, sphere correction, andcylinder correction were conducted according to JIS-B601. Thecenter-line average roughness R_(a) was taken as the value of surfaceroughness.

                                      TABLE 2                                     __________________________________________________________________________          Production                                                                           Magnetic Characteristics                                               Example for             Rotational                                                                          Surface     Overwriting                         Ferromagnetic                                                                        H.sub.c                                                                             B.sub.m                                                                             r3000/H.sub.c                                                                      Hysteresis                                                                          Roughness                                                                          Output                                                                            C/N                                                                              Character-                          Metal Particle                                                                       (Oe)                                                                             SQ (G)                                                                              SFD                                                                              (%)  Loss Integral                                                                       (nm) (dB)                                                                              (dB)                                                                             istics                        __________________________________________________________________________    Example 1-1                                                                         1-1    2010                                                                             0.82                                                                             3730                                                                             0.45                                                                             0.48 1.12  2.5  2.6 3.5                                                                              1.5                           Example 1-2                                                                         1-2    2230                                                                             0.83                                                                             4120                                                                             0.44                                                                             0.45 1.05  2.6  3.5 4.8                                                                              2.8                           Example 1-3                                                                         1-3    2470                                                                             0.82                                                                             4450                                                                             0.42                                                                             0.40 0.99  2.6  4.8 4.6                                                                              4.5                           Example 1-4                                                                         1-4    2320                                                                             0.80                                                                             4360                                                                             0.42                                                                             0.46 1.10  2.7  4.2 5.3                                                                              4.1                           Comparative                                                                         2-1    1740                                                                             0.78                                                                             3400                                                                             0.56                                                                             0.63 1.26  3.2  0.0 -0.5                                                                             1.5                           Example 1-1                                                                   Comparative                                                                         2-2    1930                                                                             0.77                                                                             3620                                                                             0.57                                                                             0.75 1.25  3.1  0.8 0.2                                                                              5.7                           Example 1-2                                                                   Comparative                                                                         2-3    2120                                                                             0.76                                                                             3760                                                                             0.58                                                                             0.71 1.28  3.2  1.0 0.0                                                                              6.9                           Example 1-3                                                                   Comparative                                                                         3-1    1880                                                                             0.83                                                                             3740                                                                             0.54                                                                             0.75 1.40  3.5  0.2 -1.0                                                                             3.2                           Example 1-4                                                                   Comparative                                                                         3-2    2300                                                                             0.81                                                                             3950                                                                             0.61                                                                             0.72 1.36  3.3  0.9 0.7                                                                              8.5                           Example 1-5                                                                   __________________________________________________________________________

PRODUCTION EXAMPLE 4 Formation of Nuclear Crystals

Into a 2-liter sealable glass container was introduced 500 ml of 2Maqueous FeCl₃ solution. To the solution was added, with stirring, 500 mlof 5.94N aqueous NaOH solution over a period of 5 minutes. Aftercompletion of the addition, the mixture was stirred for further 20minutes, and the container was then sealed tightly.

The container was placed in an oven preheated at 100° C. and kepttherein for 72 hours. Thereafter, the container was quenched withrunning water. The reaction mixture was separated into portions andcentrifuged with a centrifugal separator at 15,000 rpm for 15 minutes.The resulting supernatant was discarded. Distilled water was added tothe residue to redisperse the same. The dispersion was centrifuged againand the resulting supernatant was discarded. This water washingoperation using a centrifugal separator was conducted three times. Theresulting precipitate of hematite particles (average particle diameter,about 80 nm) which had undergone the water washing was dried. To 50 g ofthe dry particle was added 5 ml of distilled water. This mixture wastreated with a mortar for 30 minutes to pulverize the particles. Thepulverized particles were washed out of the mortar and put into a beakerusing 500 ml of distilled water. The mixture was separated into 100 mlportions, which were respectively placed into 200-ml mayonnaise bottlescontaining steel beads and treated for 10 hours to disperse theparticles. The resulting dispersions were collected, and the mayonnaisebottles were washed with distilled water to recover the remainingdispersions. Distilled water was added to the collected dispersion to atotal volume of 1,200 ml, and this diluted dispersion was treated withultrasonic for 30 minutes to further disperse the particles.

The ultrasonic-treated dispersion was separated into portions andcentrifuged at 10,000 rpm for 30 minutes. The resulting supernatant,which contained ultrafine hematite particles (average particle diameter,about 70 Å) dispersed therein, was taken out as a solution of nuclearcrystals. This solution had an iron concentration of 2,000 ppm.

Crystallite Size Control of Spindle-shaped Monodisperse Hematite

Into a reactor equipped with a stirrer was introduced 180 ml of 1 mol/lferric nitrate. The solution was cooled to 5° C. Thereto was added, withstirring, 180 ml of 2.4 mol/l sodium hydroxide solution over a period of5 minutes. After the addition, stirring was continued for further 5minutes. Subsequently, 180 ml of the solution of nuclear crystals wasadded, and the contents were stirred for 10 minutes. To each of 60 mlportions taken from the resulting mixture were added 10 ml of 0.048mol/l NaH₂ PO₄ as shape control ions and 10 ml of H₂ O. The containerswere sealed and kept for 72 hours in an oven preheated at 120° C.Thereafter, the containers were quenched with running water. Thereaction mixture was centrifuged with a centrifugal separator at 18,000rpm for 15 minutes, and the resulting supernatant was discarded.Distilled water was added to the residue to redisperse the same. Thedispersion was centrifuged again and the resulting supernatant wasdiscarded. This water washing operation using a centrifugal separatorwas conducted three times. Subsequently, 1M ammonia water was added tothe residue to redisperse the same. This dispersion was centrifuged andthe resulting supernatant was discarded. Distilled water was added tothe residue to redisperse the same. The dispersion was centrifuged againand the resulting supernatant was discarded. This water washingoperation using a centrifugal separator was conducted three times. Partof the reaction product was taken out and dried. The dry particles wereexamined with a transmission electron microscope. As a result, it wasfound that the particles obtained were α-Fe₂ O₃ particles having anaverage long axis length of 70 nm and an acicular ratio (long axis/shortaxis) of 4.5 and having an excellent particle size distribution with avariation of long axis length (standard deviation of long axislength)/(average long axis length)! of 7%.

The spindle-shaped monodisperse hematite obtained was dispersed intodistilled water in such an amount as to result in a hematiteconcentration of 2%. Cobalt sulfate was added thereto in an amount of10% in terms of Co atom amount based on the amount of the Fe atomscontained in the hematite. The mixture was sufficiently stirred. Ammoniawater was added to this suspension with stirring while monitoring the pHof the suspension to adjust the pH to 8.0. Thus, a Co compound wasdeposited on the hematite surface. Thereafter, sodium aluminate wasadded to the suspension with stirring in an amount of 8% in terms of Alatom amount based on the amount of the Fe atoms contained in thehematite, and dilute sulfuric acid was then added to adjust the pH ofthe suspension to 6.5. Further, an yttrium nitrate solution was added tothe suspension with stirring in an amount of 6% in terms of Y atomamount based on the amount of the Fe atoms contained in the hematite,and ammonia water was added to adjust the pH of the suspension to 7.5.The suspension was then filtered and washed with distilled water toremove impurities. The surface-treated spindle-shaped hematite obtainedwas compacted into cylindrical granules by passing the hematite throughcompaction plates having a diameter of 3 mm, and then dried.

A 500 g portion of the surface-treated spindle-shaped monodispersehematite was placed in a stationary reducing furnace, and annealed innitrogen at 650° C. for 1 hour. Thereafter, the temperature was loweredto 400° C., and a hydrogen/CO═30/70 mixed gas was introduced in place ofnitrogen to conduct reduction for 1 hour. Nitrogen was then introducedin place of the mixed gas. After the temperature was elevated to 450°C., pure hydrogen was introduced in place of nitrogen to conductreduction for 5 hours. Nitrogen was introduced in place of pure hydrogento cool the contents to room temperature, and an air/nitrogen mixtureregulated to have an oxygen concentration of 0.5% was introduced toconduct gradual oxidation at 50° C. or lower while monitoring thetemperature of the metal powder. After heat generation ended, the oxygenconcentration was adjusted to 1% to continue gradual oxidation for 10hours. Thereafter, a vapor of distilled water was introduced togetherwith air in such an amount as to result in a water amount of 1% based onthe amount of the metal particle to humidify and stabilize the particle.

PRODUCTION EXAMPLE 5

The hematite obtained in Production Example 4 was dispersed intodistilled water in such an amount as to result in a hematiteconcentration of 2%. Sodium aluminate was added thereto with stirring inan amount of 8% in terms of Al atom amount based on the amount of the Featoms contained in the hematite, and dilute sulfuric acid was then addedto adjust the pH of the suspension to 6.5. Further, a neodymium nitratesolution was added to the suspension with stirring in an amount of 5% interms of Nd atom amount based on the amount of the Fe atoms contained inthe hematite, and ammonia water was then added to adjust the pH of thesuspension to 7.8. The suspension was filtered and washed with distilledwater to remove impurities. The surface-treated spindle-shaped hematiteobtained was compacted into cylindrical granules by passing the hematitethrough compaction plates having a diameter of 3 mm, and then dried.

The surface-treated hematite was treated under the same conditions as inProduction Example 4 to obtain a ferromagnetic metal particle.

Magnetic characteristics of the ferromagnetic metal particles obtainedin Production Examples 4 and 5 were determined with a vibrating samplemagnetometer (manufactured by Toei Kogyo K.K.) at an external magneticfield of 10 kOe. Each ferromagnetic metal powder obtained wasphotographed with a high-resolution transmission electron microscope todetermine the average long axis length, average acicular ratio, andcrystallinity of the ferromagnetic metal particles and the averageacicular ratio of the crystallites. Further, the specific surface areaof each powder was measured with Quantasorb (manufactured byQuantachrome Inc.) after 30-minute dehydration at 250° C. in nitrogen.The results obtained are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________             Antisintering                                                                 Agent           Ferromagnetic Metal Particle                              Element                                                                             Rare          Average                                                                            Average    Average                              Production                                                                         Added earth                                                                             H.sub.c                                                                          σ.sub.s                                                                     S.sub.BET                                                                        Long Axis                                                                          Acicular   Acicular Ratio                       Example                                                                            Co  Al                                                                              element                                                                           (Oe)                                                                             emu/g                                                                             m.sup.2 /g                                                                       Length                                                                             Ratio                                                                              Crystallinity                                                                       of Crystallites                      __________________________________________________________________________    4    5.0 8.0                                                                             Y 6.0                                                                             2050                                                                             145.8                                                                             48.5                                                                             42   3.3  75    3.0                                  5    0.0 8.0                                                                             Nd 5.0                                                                            1975                                                                             138.3                                                                             45.2                                                                             45   3.8  70    3.1                                  __________________________________________________________________________

EXAMPLE 2

Tapes were produced under the following conditions using theferromagnetic metal particles obtained in Production Examples 4 and 5,and examined for magnetic characteristics and electromagneticcharacteristics.

    ______________________________________                                        (Composition for Upper-Layer Formation)                                       Ferromagnetic metal powder (Table 4)                                                                    100    parts                                        Binder resin                                                                  Vinyl chloride copolymer  12     parts                                        (--SO.sub.3 Na group content, 1 × 10.sup.-4 eq/g;                       degree of polymerization, 300)                                                Polyester polyurethane resin                                                                            4      parts                                        (neopentyl glycol/caprolactonepolyol/MDI =                                    0.9/2.6/1 (by mol)                                                            --SO.sub.3 Na group content, 1 × 10.sup.-4 eq/g)                        α-Alumina (average particle size, 0.1 μm)                                                      3.5    parts                                        Carbon black              0.5    parts                                        (average particle size, 80 nm)                                                Butyl stearate            1      part                                         Stearic acid              2      parts                                        Methyl ethyl ketone/cyclohexanone (1:1)                                                                 200    parts                                        mixed solvent                                                                 (Composition for Nonmagnetic-Layer Formation)                                 Spherical titanium oxide  80     parts                                        (average particle diameter, 0.025 μm;                                      treated with alumina;                                                         BET specific surface area, 60 m.sup.2 /g;                                     pH, 7.5)                                                                      Carbon black              20     parts                                        (average primary particle diameter, 16 nm;                                    DBP absorption amount, 80 ml/100 g;                                           BET specific surface area, 250 m.sup.2 /g;                                    pH, 8.0)                                                                      Binder resin                                                                  Vinyl chloride-vinyl acetate-vinyl                                                                      10     parts                                        alcohol copolymer                                                             (content of polar group --N.sup.+ (CH.sub.3).sub.3 Cl.sup.-,                  5 × 10.sup.-6 eq/g;                                                     monomer composition ratio, 86/13/1;                                           degree of polymerization, 400)                                                Polyester polyurethane resin                                                                            8      parts                                        (backbone: 1,4-BD/phthalic acid/HMDI =                                        2/2/1 (by mol);                                                               molecular weight, 10,200;                                                     hydroxyl group content, 0.23 × 10.sup.-3 eq/g;                          --SO.sub.3 Na group content, 1 × 10.sup.-4 eq/g)                        Butyl stearate            1      part                                         Stearic acid              2.5    parts                                        Methyl ethyl ketone/cyclohexanone (1:1)                                                                 200    parts                                        mixed solvent                                                                 ______________________________________                                    

With respect to each of the composition for non-magnetic-layer formationand the composition for magnetic-layer formation, the ingredients werekneaded with a kneader and then dispersed with a sand grinder. To theresulting dispersions was added a polyisocyanate in an amount of 5 partsfor the coating composition for nonmagnetic-layer formation and in anamount of 6 parts for the coating composition for magnetic-layerformation. To each dispersion was further added 20 parts of a methylethyl ketone/cyclo-hexanone (1:1) mixed solvent. These dispersions werefiltered through a filter having an average opening diameter of 1 μm toprepare a coating composition for nonmagnetic-layer formation and acoating composition for magnetic-layer formation.

The two coating compositions obtained were applied on a 7 μm-thickpoly(ethylene terephthalate) support by wet-on-wet simultaneousmulti-layered coating as follows. The coating composition fornonmagnetic-layer formation was applied first at a dry thickness of 1.5μm, and the coating composition for magnetic-layer formation was appliedimmediately thereafter on the still wet nonmagnetic layer coating insuch an amount as to obtain a 0.15 μm-thick magnetic layer. While thetwo coating layers were still in a wet state, magnetic orientation ofthe ferromagnetic metal particles was performed with a rare earth magnet(surface flux density, 5,000 G) and a solenoid magnet (surface fluxdensity, 5,000 G). The coating was then dried. Subsequently, calenderingwas conducted with a 7-roll calender comprising metal rolls at a rolltemperature of 90° C. to obtain a magnetic recording medium as a web.The web was slit into an 8-mm width. Thus, 8-mm video tape samples wereproduced. The samples were evaluated in the same manner as in Example 1,and the results obtained are shown in Table 4. As a standard for theexamination of electromagnetic characteristics, Super DC Tape(manufactured by Fuji Photo Film Co., Ltd.) was used.

                                      TABLE 4                                     __________________________________________________________________________          Production                                                                    Example                                                                       for Ferro-                                                                         Magnetic Characteristics                                                 magnetic              Rotational                                                                          Surface     Overwriting                           Metal                                                                              H.sub.c                                                                             B.sub.m                                                                             r3000/H.sub.c                                                                      Hysteresis                                                                          Roughness                                                                          Output                                                                            C/N                                                                              Character-                            Particle                                                                           (Oe)                                                                             SQ (G)                                                                              SFD                                                                              (%)  Loss integral                                                                       (nm) (dB)                                                                              (dB)                                                                             istics                          __________________________________________________________________________    Example 2-1                                                                         4    2230                                                                             0.83                                                                             4350                                                                             0.42                                                                             0.36 0.85  2.5  4.5 5.5                                                                              2.5                             Example 2-2                                                                         5    2170                                                                             0.84                                                                             4160                                                                             0.44                                                                             0.41 0.88  2.6  3.9 4.8                                                                              2.6                             __________________________________________________________________________

As is apparent from the results of Tables 2 and 4, in each of theExamples of the present invention, r3000/H_(c) was low, and the tape hasa satisfactory coercive force distribution. Further, the rotationalhysteresis integral determined through a magnetic torque measurement issmall, and the reversal of magnetization occurred by a mechanism closesto simultaneous rotation.

The ferromagnetic metal particles used in the Examples of the presentinvention, in which particles each had a crystallinity of 30% or higher,had nearly the same average long axis length and acicular ratio. Incontrast, the ferromagnetic metal powders used in the ComparativeExamples, in which particles each had a crystallinity of below the lowerlimit specified in the present invention, each had an average long axislength outside the range specified in the invention and had an increasedcoefficient of variation of long axis length. It is presumed that theincreased coefficients of variation in the Comparative Examples resultedin the increased values of r3000/H_(c).

Even an extraordinarily fine ferromagnetic metal powder having anaverage long axis length of from 30 to 80 nm can be made to be amagnetic particle having a high coercive force and an excellent coerciveforce distribution by using a starting material having uniformity inparticle size and by controlling the number of metal nuclei during theformation thereof to regulate the metal particles so as to have acrystallinity of from 30 to 100%. The magnetic recording mediumcontaining this magnetic particle has a high output, a high C/N, andexcellent overwriting characteristics.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A magnetic recording medium comprising anonmagnetic support having thereon a magnetic layer containing at leastferromagnetic particles, wherein said ferromagnetic particles have acoercive force of from 1,700 to 3,000 Oe, an average long axis length offrom 30 to 80 nm, an average acicular ratio of from 2.0 to less than5.0, and a crystallinity of from 30 to 100%.
 2. The magnetic recordingmedium as claimed in claim 1, wherein the ferromagnetic metal particleshave a saturation magnetization σ_(s) of from 125 to 165 emu/g.
 3. Themagnetic recording medium as claimed in claim 1, wherein theferromagnetic metal particles comprise iron and Co and have a coerciveforce of from 1,700 to 2,800 Oe.
 4. The magnetic recording medium asclaimed in claim 1, which has a nonmagnetic layer mainly comprisinginorganic nonmagnetic particles and a binder between the nonmagneticsupport and the magnetic layer.